Synthesis of [3a,7a]-Dihydroindoles by a Tandem Arene Cyclopropanation/3,5-Sigmatropic Rearrangement Reaction

Donor/acceptor carbenes
provide a powerful platform for building
molecular complexity, but the majority of their reactions have been
limited to aryl and vinyl donor groups. We found that a N-containing
donor/acceptor carbene precursor, 4-phthalimido-N-methanesulfonyl-1,2,3-triazole, reacts with unactivated arenes resulting
in a mixture of [3+2]-cycloadducts, [3a,7a]-dihydroindoles, and formal
C–H functionalization products in up to 82% yield upon heating.
We also demonstrate that the formal C–H functionalization products
arise from ring-opening of the [3+2]-cycloadducts. Computational studies
suggest that the formal cycloaddition process takes places through
a tandem arene cyclopropanation/6π electrocyclization/6π
electrocyclic ring-opening/3,5-sigmatropic rearrangement reaction,
which also accounts for the distinctive regioselectivity of the formal
cycloaddition reaction.